Fiber-reinforced composite materials, particularly carbon-fiber-reinforced composite materials, which comprise carbon fiber and a matrix resin, have wide applications not only in the field of sporting goods such as golf clubs, tennis rackets and fishing rods, but also in the fields of structural materials for aircrafts or vehicles and of reinforcement of concrete structures, because of their superior mechanical properties. In recent years, carbon-fiber-reinforced composite materials have been used for cases of electrical/electronic equipment such as note-type personal computers and video cameras, because of the electrical conductivity of carbon fiber and their superior electromagnetic shielding properties and mechanical properties, contributing to providing thinner-wall casings or lighter-weight equipment.
In one of such applications of fiber-reinforced composite materials, that is, in the application in the field of structural materials for aircrafts or vehicles or of building materials, it is strongly required that fiber-reinforced composite materials have flame retardance, because it is very dangerous that structural materials catch fire and burn and emit toxic gases.
In the applications in the field of electrical/electronic equipment, it is also required that the materials have flame retardance, because accidents, such as ignition or burning of equipment casings or parts, may occur when the materials are exposed to heat developed inside the equipment or high temperatures outside the equipment.
Traditionally, halogen flame-retardants have been widely used to impart flame retardance to fiber-reinforced composite materials. Specifically, there are disclosed flame-retardant epoxy resin compositions using, as a flame-retardant, a brominated epoxy resin or a brominated epoxy resin together with antimony trioxide (e.g. JP Patent Publication (Kokoku) Nos. 59-2446B (1984) and 59-52653B (1984), JP Patent Publication (Kokai) Nos. 6-206980A (1994) and 9-278914A (1997)). There are also disclosed flame-retardant epoxy resin compositions and prepregs using, as a flame-retardant, an organic halogen compound such as hexabromobenzene (e.g. JP Patent No. 3216291).
These halogen flame-retardants produce a high flame-retardant effect, but on the other hand, they can sometimes generate a noxious gas, such as hydrogen halide or organic halogen compound, during the time that composite materials catch fire and the fire is extinguished. And it is known that incineration of plastic materials containing a halogen flame-retardant at insufficiently high temperatures emits dioxins, which are carcinogens. Furthermore, antimony trioxide, which is used together with a halogenated flame-retardant, is hazardous due to its irritant action, and care must be taken when handling it. Thus, there have been demands in recent years that a certain level of flame retardance should be achieved without using a halogen flame-retardant or antimony trioxide.
Further, halogen flame-retardants have a halogen atom as an integral part of molecule, and thus, their specific gravity itself is as high as about 1.9, while that of ordinary cured epoxy resins is about 1.2 (specific gravities herein described are all those measured at 25° C.). Furthermore, the specific gravity of antimony trioxide, which is used together with a halogen flame-retardant, is as high as 5.2. Thus, a cured resin obtained by curing a resin composition having any of these flame-retardants added has a higher specific gravity than a cured resin obtained by curing a resin composition having none of such flame-retardants added. This, in general, results in increase in the specific gravity of fiber-reinforced composite materials produced using, as a matrix resin, a resin composition having any of these flame-retardants added and may cause the problem of being unable to make full use of the characteristics of fiber-reinforced composite materials, light weight and high stiffness.
In the meantime, as a technique for providing halogen-free flame-retardant epoxy resin compositions, there is disclosed a technique where a matrix resin for fiber-reinforced composite materials is made up of: epoxy resin, metal oxide and thermoplastic resin having a glass transition temperature of 120° C. or higher (e.g. JP Patent Publication (Kokai) No. 11-147965A (1999)). This technique has the advantage of not emitting a halogen gas, but on the other hand, it requires 20 parts or more of metal oxide to be added to achieve sufficient flame retardance. Resin compositions containing a large amount of such a flame retardant have so high viscosity that they are hard to impregnate into reinforcing fiber, which is likely to have a detrimental effect on handleability of prepregs, to allow the formation of voids in the molded composite materials, and to cause deterioration in physical properties of the composite materials, particularly in tensile properties.
Further, metal oxides have a high specific gravity, like halogen flame-retardants. For example, magnesium oxide has a specific gravity of 3.2 or more, and thus, addition of such a compound as a flame-retardant causes the problem of increasing the specific gravity of the resultant resin compositions and fiber-reinforced composite materials, just like the problem with halogen flame-retardants.
As described so far, in the present state of art, it is hard to obtain a light-weight non-halogen flame-retardant epoxy resin composition which allows fiber-reinforced composite materials to have superior mechanical properties.
For casings or members of electrical/electronic equipment and information equipment such as note-type personal computer, cellular phone, mobile information terminal and digital camera, thermoplastic resins have been used. In recent years, with the quick spread of such equipment, there have been increasing demands for thin and light weight products in the market. And with the increase in such demands, casings and internal members that constitute the products have been required to be not only of thin wall and light weight, but of high strength and high stiffness.
To meet this requirement, magnesium alloys have been put to practical use. But on the other hand, there have been increasing requirement for high stiffness, and to meet this increasing requirement, consideration has been given to using metallic materials having high stiffness, such as aluminum alloys. From these metallic materials, however, members or products having a complicated shape are hard to produce in large quantity and easily, and at the same time, due to high specific gravity of such metallic materials, the requirement of light weight has not been satisfied yet.
On the other hand, fiber-reinforced composite materials (FRPs), each of which is made up of matrix resin and continuous reinforcing fiber arranged in the matrix resin, particularly carbon-fiber-reinforced composite materials (CFRPs), in which carbon fiber is used as the reinforcing fiber, have been widely used, as materials excellent in mechanical properties and light-weight, in the production of various kinds of parts or structures. These FRPs are, however, poorly suited to producing parts or structures having a complicated shape in a single molding step; therefore, in the above described applications, the production process requires the steps of: forming members of FRP; and integrate the formed members with other members.
Materials used for applications, such as electrical/electronic equipment or information equipment, are sometimes strongly required to have flame retardance so as to prevent accidents such as ignition or burning of equipment casings or parts, which may occur when the casings or parts are exposed to heat developed inside the equipment or high temperatures outside the equipment. As thermoplastic resin materials used for such application, those blended with various types of flame-retardants are generally known. For example, there are disclosed conductive casings for electronic equipment which are produced by injection-molding resin compositions composed of carbon fiber, semiaromatic polyamide, aliphatic polyamide, and red phosphorus as a flame-retardant (e.g. JP Patent Publication (Kokai) No. 10-120798 (1998)).
As described above, to impart flame retardance to fiber-reinforced composite materials, halogen flame-retardants have been widely used. For example, there are disclosed carbon-fiber-reinforced composite materials in which brominated epoxy resin and antimony trioxide as a flame-retardant are used (e.g. JP Patent Publication (Kokai) No. 9-278914 (1997)). This flame-retardant, however, has the problem of its use for the above described applications being restricted due to it noxiousness to the environment and the human body.
There are also disclosed fiber-reinforced composite materials in which an epoxy resin composition is used as a matrix resin and magnesium oxide or aluminum oxide as a non-halogen flame-retardant (e.g. JP Patent Publication (Kokai) No. 11-147965 (1999)). However, to achieve sufficient flame retardance by this known technique, a large amount of flame-retardant needs to be added. Addition of a large amount of flame-retardant increases the viscosity of the resin composition, thereby giving rise to the problem of causing molding faults, such as void formation, which leads to deterioration in mechanical properties. Further, since such a flame-retardant has a high specific gravity, addition of a large amount of flame-retardant gives rise to the problem of failing to impart superior light weight to the final composite materials.
As described so far, in the present state of art, moldings in which members of FRP are integrated do not satisfy not only mechanical properties and light weight, but also superior flame retardance, which are required when they are used for the above described applications.
The present invention has been made in the light of the above described problems with prior art. Accordingly, a primary object of the present invention is to provide a light-weight fiber-reinforced composite material which has superior flame retardance and mechanical properties and never emits a halogen gas when it is incinerated, and a prepreg and an epoxy resin composition both suited to obtain such a fiber-reinforced composite material.
Another object of the present invention is to provide an integrated molding in which not only high mechanical properties and light weight, but also superior flame retardance is accomplished without using a halogen flame-retardant and which is suitable for use as a casing for electrical/electronic equipment.